https://www.nature.com/articles/d41586-022-04164-8.pdf

Table of Contents

Sangwon Suh & André Bardow

Modelling reveals that the carbon emissions associated with plastics could be negative by 2100 under a strict set of technological and socio-economic conditions — including increased recycling and plant-derived production.

The direct effect of plastics on the marine ecosystem has attracted global attention. However, the production and disposal of plastics are also a concern, because these processes release more climate-warming gases annually than does global aviation. And these emissions are increasing: the growing global appetite for plastics is expected to result in a doubling of their associated carbon emissions by 2050. Such an increase would prevent us from achieving netzero emissions, a target that is widely held to be necessary to protect the planet’s ability to support life. On page 272, Stegmann et al provide a road map for avoiding this future by examining the entire life cycle of plastics in the context of various strategies for mitigating climate change.

The good news is that it is technically possible to produce and dispose of plastics with net-zero — or even negative — carbon emissions, using technologies that already exist. However, the future deployment of such approaches has not been modelled with respect to socio-economic and technological factors, such as demography, income, the prices of energy and carbon, and the efficiencies of recycling technologies. These factors are crucial, because they affect the economic competitiveness of the technologies, as well as their carbon balance.

Stegmann and colleagues’ study fills this gap by modelling the future of the plastics industry using a baseline ‘middle of the road’ socio-economic pathway. Their model then considers how specific changes to this pathway could lead to plastics having negative carbon emissions, while limiting the global mean temperature increase to 2°C by the end of the century.

So how does it work? The problem of global climate change is largely about where to store carbon among Earth’s four compartments: the atmosphere, biosphere, hydrosphere and geosphere. Storing carbon in the atmosphere alone causes climate change. But there is a sizeable fifth compartment in which carbon can be stored: the technosphere (comprising all the technological objects manufactured by humans, as well as our social and professional systems). Plastics are made mainly from carbon that comes from crude oil and natural gas, but they can also be produced using biomass, which draws carbon from the atmosphere. Through the conversion of biomass to plastics, this carbon is transferred from the biosphere to the technosphere, where it remains for a long time, either in use (for example, in building materials) or in secure landfill.

Therefore, Stegmann and colleagues demonstrate that, by substituting oil with biomass as a feedstock for plastics, and using our enormous appetite for plastics to create a vessel for storing carbon, humans could use the global production of plastics to remove carbon from the atmosphere. They show that increased recycling would further reduce the reliance of future plastics on biomass feedstock, energy and space for landfill.

The true value of the study lies in its ability to offer insights into the socio-economic and technological conditions under which plastics turn into a carbon sink. To achieve this, the authors used a framework developed previously by members of the same team to model the life cycle of plastics, from production to disposal. They first assessed how this model behaved in the baseline scenario, and examined how it would change with an increase in the price of carbon emissions, eliciting a global mean temperature increase of up to 2 °C. They then looked at other changes that promote circular-economy strategies, such as recycling and renewable-energy use, as well as a scenario that prioritizes biomass use in this circular economy.

Through their analysis, Stegmann et al. show that plastics could achieve negative carbon emissions by the end of the century (Fig. 1), but only under a certain set of conditions — and they are tough to meet. They include implementing a globally uniform carbon-pricing scheme; offering up to 30% subsidies for companies using biomass to produce plastics; and mandating that the yields of key recycling technologies are increased by up to 20%. Each of these conditions is a tall order on its own. In that sense, more than anything else, the study highlights the magnitude of challenge that lies ahead.

As the authors point out, however, their results should be interpreted with caution. For example, their baseline scenario is intended to represent a ‘business as usual’ pathway, in which future behaviours largely follow historical trends. But many of the conditions they impose, including globally uniform carbon pricing, fall a long way outside this pathway. In our view, the friction between these two sets of assumptions — that nothing will change and everything will change — limits the extent to which policy-relevant interpretations can be drawn directly from the authors’ results.

Future research will need to address the unintended impacts of storing carbon as plastics in the technosphere. The authors’ scenario assumes that plastics production will double in volume by 2050, which will help to turn the industry carbon negative by increasing the carbon stock in the technosphere. Increased throughput, however, might exacerbate other problems, including the effect that plastics have on marine life. The extra demand for biomass could also escalate the competition for arable land, which is already stressed by the production of feed, fuel and food, and could lead to increased use of agrochemicals and fertilizer. Finally, to make the authors’ recycling and biomass goals attainable, materials and processes will have to be improved. This includes redesigning polymers to make them more amenable to recycling than those currently in use, and updating the processes for both recycling and biomass conversion. Such engineering-level details will need to be incorporated into future models.

It seems plausible that plastics could become a carbon sink in future. But will they? In our opinion, the answer hinges mainly on society’s ability to create a socio-economic and political landscape that facilitates the transition, rather than on the development of necessary technologies. It remains unclear why global efforts can facilitate the conditions necessary to overcome some global environmental problems, such as ozone-layer depletion, but not others — yet. What can science do to help us understand the barriers to creating such conditions? Answers to these questions will be crucial for turning possibility into reality, and Stegmann and colleagues’ study is a key step in this process.

Sangwon Suh is in the Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, California 93106, USA.
André Bardow is in the Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland.
e-mails: suh@bren.ucsb.edu; abardow@ethz.ch

Figure 1 | Carbon balance of the plastics industry. Stegmann et al.2 modelled the life cycle of plastics (from production to disposal) in the context of
four scenarios for climate-change mitigation. Their baseline scenario was a ‘middle of the road’ socio-economic pathway (see go.nature. com/3uvdbgs). They then examined the effects of increasing the price of carbon emissions,
incentivizing recycling and renewable-energy use, and prioritizing the use of biomass as a feedstock for plastics production. In two of the four scenarios, the plastics industry is forecast to have net negative carbon emissions before 2100. (Adapted from Fig. 3 of ref. 2.)